1. Field of Invention
The present invention relates to a method and system for communication over wireless local area networks using orthogonal frequency division multiplexing.
2. Description of the Prior Art
Future wireless local area networks (WLANs) will use orthogonal frequency division multiplexing (OFDM) as the modulation method. OFDM is well suited to the requirements of localised, broadband communications and has been shown to operate at raw data rates of 40 Mbit/s in demonstration conditions.
When a signal passes through a channel to a receiver, distortions due to the channel are introduced to the signal. As well as the channel-induced distortions, frequency differences in the transmitter and receiver RF local oscillators and sample clocks due to component tolerances introduce, respectively, frequency and timing errors. Each of the time varying frequency, phase and timing errors and the packet start require estimation by the receiver. A number of methods of estimating these parameters in OFDM systems have previously been proposed. Methods have been proposed based on pilot symbols and based on null symbols. Assuming that initial synchronisation has been achieved, methods to track these parameters from OFDM data have also been previously proposed. Alternative tracking and/or adaptive channel estimation methods have been proposed which are based on embedded pilot tones. A disadvantage of the currently proposed systems with currently proposed pilot symbols, null symbols and cyclic prefixes is the overhead needed to obtain initial estimates of and track the time varying time, frequency and phase errors.